When an optical fiber having a low optical loss is used in a long-distance optical communication network, a relay distance may be extended, making it possible to significantly lower construction costs of the entire transmission network. The loss of an optical fiber mainly includes Rayleigh scattering caused by density fluctuations in glass, Brillouin scattering generated when optical signals and interior materials cause a minute change in refractive index while being acoustically shaken, Raman scattering generated by input optical signals due to an interaction between the optical signals and molecular vibrations in glass, and an optical loss caused by infrared absorption. Among the factors, Rayleigh scattering is of the greatest importance.
It is now known that when the composition of a core corresponding to an optical signal transmission area is a pure-silica-core fiber (PSCF) corresponding to pure silica glass, a value of an optical loss is substantially not more than 0.15 dB/km. GeO2 is added to SiO2 to increase refractive index in a core of a general communication optical fiber, and as the amount of GeO2 increases, refractive index increases but density fluctuation also increases so that a relatively high scattering loss appears as compared with a pure-silica-core fiber (PSCF).
It is true that the PSCF has a very low optical fiber loss, but there are many restrictions in realizing it. In order that most input optical signals may be transmitted through a core area in the PSCF, there should be a suitable refractive index difference between the core area and a cladding area surrounding the core area. In a single mode optical fiber, the value corresponds to approximately 0.33% to 0.35% of the refractive index of silica glass. The PSCF includes an F-doped cladding layer that may refractive index to adjust a negative refractive index of a large absolute value. When the PSCF has a negative refractive index of a large absolute value, dispersion and a cut-off wavelength value are greatly influenced by a ratio D/d of the diameter D of the cladding to the diameter d of the core. Accordingly, the diameter of the cladding should be sufficiently large, and thus there is a restriction in increasing the size of an optical fiber preform.
Further, there is a restriction in drawing the PSCDF at a high speed because refractive index changes and a Rayleigh scattering loss increases when a drawing tension is applied. Accordingly, the productivity of optical fibers is very low, which increases production costs. In addition, in relation to the curving characteristics of the PSCF, because a change in relative refractive index due to a stress (outer curved surface-tensile stress, inner curved surface-compressive stress) applied as the PSCF is curved is large, there is a limit in maintaining a low refraction loss. Accordingly, in spite of its very low loss value, the PSCF is not widely used in a communication network due to its difficulty in manufacturing, inferior curving characteristics, and high manufacturing costs, and is used only for the special purposes such as a submarine cable. Accordingly, if an optical fiber having a core composition (SiO2—GeO2) widely used in a generic-purpose optical communication network has a low loss and curving characteristics, it may be immediately applied to expansion of an existing communication network and may significantly lower construction costs of a new communication network.
A Rayleigh scattering loss that is the most important of the optical fiber losses is closely relevant to a thermal history that is undergone in an optical fiber drawing process. In particular, it is relevant to a phase transition process that occurs in a cooling process, and the phase transition process has a very close relation to a fictive temperature Tf that is a temperature at which the step is changed to a frozen-in step in which density fluctuations are not present in the presence of fluidity. The relationship between a fictive temperature Tf of glass and a Rayleigh scattering loss is well established through various documents, and it is known that the Rayleigh scattering loss decreases as the fictive temperature Tf of glass decreases. The fictive temperature Tf of glass is relevant to a cooling rate of glass, and as the cooling rate decreases, the fictive temperature Tf of glass decreases.
Accordingly, theoretically, if optical fiber drawing temperature is sufficiently low or optical fiber drawing speed is low, optical fiber cooling speed is low and glass fictive temperature Tf is low so that a Rayleigh scattering loss is so low to obtain an optical fiber of a low loss. However, if the temperature of an optical fiber furnace decreases, an optical fiber drawing tension significantly increases to apply an excessive stress to an optical fiber and cause a change in refractive index, thereby increasing loss. Accordingly, considering the fact, both a low drawing temperature and a low drawing speed are necessary in order to obtain optical fiber characteristics of a very low loss. The operation temperature of a general optical fiber furnace is not less than 2250° C. and the drawing speed of the optical fiber furnace is about 30 m/s. Accordingly, even though an optical fiber of a very low loss may be obtained through a method of drawing an optical fiber at a very low drawing temperature and at a very low drawing speed, productivity deteriorates severely and it is impossible to manufacture an optical fiber of a competitive price.